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Basics - Poster 8
Radiation Dose Units
When making measurements in any field of science , it is important to be clear about the quantities that are being measured , eg length and time , and the units of measurements , eg metres or seconds . The same is true for radiation dose units .
The simplest quantity of radiation dose is the Absorbed Dose . This is a measure of the energy deposited by ionising radiation per unit mass of matter , such as tissue . It is measured in joules per
-1
kilogram ( J kg ) and the SI unit ( International System of Units ) is -1 the gray ( Gy ) where 1 Gy = 1 J kg .
Tissue Weighting Factor Some organs in the body are more susceptible to damage from ionising radiation than others . To take account of this too , tissue weighting factors ( W ) are assigned to the body organs . When all
T the tissue weighting factors are summed , it comes to 1 .
The equivalent dose to each organ is multiplied by the organ ' s tissue weighting factor – this new quantity has the unit of the Sievert ( Sv ). The sum of all tissue weighted equivalent doses is called the Effective Dose ( E ) and gives a measure of the damage to the whole body .
It is an important unit in radiotherapy , as a measure of the dose given to a treatment area , such as that to a tumour .
Radiation Weighting Factor For the same absorbed dose , some radiations cause more damage to organs and tissues than others . For example , alpha radiation , because it is densely ionising , causes 20 times as much damage to body tissue as X-rays , gammas and betas . To account for this , a Radiation Weighting factor ( W ) is applied . This
R quantity is now known as the Equivalent Dose ( H ) and the unit is
T called the Sievert ( Sv ) Equivalent dose ( Sv ) = Absorbed dose ( Gy ) x W R This allows the comparison of doses to an organ or tissue from
The Sv is a large unit of radiation dose . Therefore submultiples are often used .
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1 millisievert ( mSv ) = 10 Sv
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1 microsievert ( μSv ) = 10 Sv where the product W T H T is summed over all tissues , T ( Figures are ICRP 2007 values )
Example
Suppose just the thyroid is irradiated by beta radiation and received an average absorbed dose of 400 mGy . What is the effective dose ( E )? Equivalent dose ( H ) = 400 x 1 = 400 mSv
T Effective dose ( E ) = 400 x 0.04
= 16 mSv This dose would give rise to the same detriment as if all the organs in the body had been exposed to a total of 20 mGy .
Banana Equivalent Dose Bananas are a good source of potassium , a substance necessary for heathy operation of your body . Radioactive potassium-40 is a very small part of all potassium and makes up part of everyone ’ s exposure to natural ionising radiation . The potential dose due to eating just one average sized banana is often taken as 0.1μSv - a one-off exposure as the body excretes potassium to maintain metabolic equilibrium . Some people use this Banana Equivalent Dose ( BED ) as a simple , fun way of comparing radiation exposures from different sources . The BED is not a formally adopted unit !